Methane, CH₄

The escape of combustible gas from the earth is mentioned by Pliny, and the outbursts of flame in mines received the attention of Basil Valentine. Volta, in 1776, showed that a volume of methane required for its combustion four times the volume of oxygen that was required by an equal volume of hydrogen, and that it produced carbon dioxide when burnt. In 1785 Berthollet proved that the gas contained carbon and hydrogen; and in 1805 William Henry distinguished it from the denser ethylene.

Methane was thenceforth called light carburetted hydrogen, ethylene being heavy carburetted hydrogen. On account of its formation by the decay of vegetable matter in swamps, methane is likewise known as marsh-gas; and because it is the cause of explosions in coal-mines it is also called fire-damp (Ger. Dampf = vapour).

Origin of Petroleum. - Two theories have been proposed to account for the origin of petroleum: (i) that it owes its formation to the action of water on carbides; (ii) that it is formed by the decomposition of plant and animal remains.

1. Berthelot supposed that acetylides are formed by the action of carbon dioxide on the alkali metals at high temperature, from which acetylene is generated by the action of water; and Mendeleeff2 assumed that subterranean iron carbides have similarly given rise to petroleum. Moissan adopted Mendeleeff's view, which appears to be supported by the presence of methane in volcanic gases and liquid and solid hydrocarbons in lava, and also by the discovery of occluded hydrocarbons in meteorites.
   
   It must be pointed out, however, that the theory of the production of hydrocarbons from inorganic materials postulates the high temperature necessary for the formation of metallic carbides, and that many geologists now discard the idea that the earth was once molten.
2. The formation of methane by the decomposition of vegetable matter and of various hydrocarbons in the destructive distillation of coal are facts that suggest that petroleum generally owes its origin to an organic source.

Engler distilled fish oil under pressure, and from the crude product obtained an illuminating oil, consisting of hydrocarbons and resembling kerosene. Sadtler similarly obtained a mixture of hydrocarbons by distilling linseed oil. Phillips proved the formation of methane in the slow decay of seaweed; and Engler estimated from the annual catch of herring on the north coast of Germany that the total petroleum- supply of Galicia would be produced from fish in about 2500 years. Other possible sources of petroleum are molluscs, radiates, and diatoms. The presence of optically active hydrocarbons in petroleum makes the organic source of this product practically certain, and the occurrence of the oil in sedimentary rather than igneous rocks lends further support to the same view. There is little doubt, therefore, that petroleum owes its origin to an animal or vegetable rather than a mineral source.

There are apparently three natural sources of methane: (i) marshes, (ii) coal, (iii) petroleum springs.

1. The gas-bubbles which are seen to rise through the water of stagnant pools consists of methane and nitrogen together with a little oxygen and carbon dioxide. The methane is produced by the bacterial fermentation of the cellulose of dead plants, and the chemical reaction has been represented as follows:
   
   C₆H₁₀O₅ + H₂O = 3CH₄ + 3CO₂.
2. Coal contains adsorbed methane; and the presence of this gas in coal represents a later stage of the same kind of decomposition, which accounts for its formation in marshes.
3. Inflammable gas, consisting chiefly of methane, escapes from holes in the earth in the neighbourhood of subterranean petroleum. Thus, at Baku, on the Caspian Sea, a " holy fire " has been burning for many centuries. Gas collected at Pittsburg, in the oil district of America, is conveyed fifteen miles in pipes and used as fuel; that at Heathfield, Sussex, contains 94 per cent, of methane and 3 per cent, of ethane.

Methane occurring near to petroleum must be considered the most volatile part of the petroleum, so that the question of its origin there is the same as that of the petroleum itself.